The epithelium of the distal lung performs critical biological functions including gas exchange and providing a barrier to prevent access of deleterious airborne agents into the body. It is also the target of numerous inherited and acquired diseases, represented by over 10% of the US population suffering from chronic lung disease. Despite the importance of this tissue, there is considerable disagreement as to the cell types important for the maintenance and repair of this organ. In the trachea, basal cells are the key progenitor cell type, while clara cells are thought to be more important progenitors in distal airways, especially in mice which lack basal cells outside the trachea. Alveolar type II cells have been long established as an important progenitor population for alveoli, a paradigm repeatedly confirmed in recent studies. In addition, several groups have identified cells that bear no mature lineage markers as potential progenitors for alveolar cell types. Specifically, distal lung integri 4-expressing cells are proposed to contribute to alveolar repair after bleomycin-induced injury, while cytokeratin 5(Krt5)/p63-expressing cells have been implicated in alveolar regeneration after influenza infection. Our unpublished work demonstrates that abundant Krt5pos cells also appear after bleomycin injury, and in both injury models these cells strongly express 4. Further characterization of freshly isolated 4+ cells from uninjured lungs revealed that these cells express p63, but not Krt5. However, upon expansion ex vivo, Krt5 is upregulated, demonstrating the propensity of these cells to adopt a basal cell-like phenotype. The major goal of this proposal is to determine whether these distal p63+ 4+ are in fact the cell-of-origin for expanded post-injury Krt5+ cells and to define the extent to which these cells are responsible for alveolar repair / regeneration. We will address this hypothesis with the following Specific Aims: 1) Determine the in vivo activation response of 4+ p63+ cells to lung injury. 2) Test whether regulated expression of p63 is critical to the acquisition of a Krt5+ regenerative phenotype by distal lung 4+Krt5+ cells. Several strains of transgenic mice, lung slice culture imaging, and in vitro molecular biology techniques will be used to address these aims experimentally. These studies will result in a better understanding of how lung epithelium responds to injury and will provide insights as to how lung progenitors might be exploited / directed towards therapeutic purposes.